The need to determine the drying rate of a liquid film applied on a substrate frequently arises in a wide variety of contexts and industrial applications such as printing inks, paints, coatings, lacquers, electronics, etc. For instance, printing ink in its supply form is liquid (low viscosity liquids such as gravure or ink-jet inks or high viscosity liquids such as letterpress or lithographic inks) but changes to a solid form after its application.
This change from liquid to solid state (drying) may occur in many ways:
Evaporation drying—where the rate of drying depends on the evaporation rate of the solvents used;
Oxidation drying—where the liquid dries by oxidation as a result of chemical reaction between the liquid and oxygen from the atmosphere;
Absorption drying—where the liquid and/or solvents can penetrate into the porous structure of the substrate;
Chemical drying—different than oxidation mechanism e.g. thermosetting liquid such as inks or paints, here chemical cross linking takes place at the elevated temperatures; and
Radiation curing—where polymerization of monomers and oligomers used in the liquid takes place when the liquid ink film is irradiated with ultraviolet (UV) light or an electron beam (EB).
Additional details on the different mechanisms for liquid drying can be found for printing inks in “The Printing Ink Manual” by R. Leach and R. Pierce, 5th Edition, copyright 1993.
Typically, liquids such as printing inks have been reported to dry on the basis of a simple scale having gradations ranging from “quick or fast drying” to “medium drying” and “slow drying”. The determination of which of the three drying rate categories a particular liquid should be classified in has usually been done on the basis of visual and/or tactile inspection. In this regard a sample of the liquid is applied wet to a substrate material, typically glass or metal, and the degree of dryness is visually or physically observed over certain pre-determined time periods. The liquid is observed over a range of time between several seconds to hours in duration, at which times the visual appearance of the sample is noted and/or the surface of the sample is contacted or touched in order to determine the tackiness of the sample. Thus, a sample is determined to be either dry or still drying and is correspondingly assigned either a “quick or fast drying”, “medium drying”, or “slow drying” designation. This method is extremely subjective and non-quantitative.
The appropriate drying rate of an ink, if not known, can cause severe problems for printers. For instance, during printing with gravure or flexographic inks the ink may dry in the gravure cylinder or anilox roller cells. The dry ink can clog the cells of, for example, a printing cylinder and make printing impossible. Therefore, the drying rate of the ink is extremely important and has to be appropriately balanced to achieve the expected result, e.g. high printing speed and high quality prints. An ink drying too fast may affect ink transfer in flexographic and gravure printing since the ink will dry either too fast or too slow on the plate, rollers or cells. Further printing problems include clogged print head nozzles and poor print quality in the final product, e.g. ink jet printing.
Another method used is a Geiger Press Test wherein the resolubility of a “dry” liquid is determined by applying ink onto the drying liquid film in a simulation of stopping and re-starting a press and having the same printability as previous to the stoppage. If the “dried” film resolubilizes, the number of proofs necessary to get back to original state is noted and a drying classification and resolubility classification is determined for that liquid film. Again, this test is subjective and non-quantitative.
Moreover, it is extremely difficult or impossible to make any evaluation when the liquid dries very rapid. There are some instruments available in the market that allow for a less-subjective evaluation of liquid drying such as the PIRA ink drying time tester, the IGT drying time recorder, and the Thin Film Analyzer (TFA). These instruments are based on different principles and can be used only for high viscosity and relatively thick liquid films (e.g. paints, coatings). They cannot be used to evaluate the drying rate of low viscosity, fast drying and relatively thin liquid films.
At the present, there is no instrument capable of quantitatively characterizing the drying rate of liquids and liquid films. There is a need in the art for a technique of measuring the drying rate of liquids and/or liquid films applied or to be applied to a substrate on a more precise, quantitative, basis. Because there is no simple way to directly measure the parameter “drying rate”, it is, alternatively, necessary to measure some other physical property related to the liquidity or fluidity, that conversely relates to the degree of dryness of the liquid being measured.
The drying rate of the liquid may also affect some experimental techniques used to characterize the properties of liquids such as, e.g., dynamic surface tension (DST). The instruments that are used to measure the DST use the maximum bubble pressure (MBP), such as the BP2 tensiometer by Kruss, USA, or to measure the differential maximum bubble pressure (DMBP), such as the PC9000 by Sensadyne, USA, are techniques which utilize a single or multiple capillaries immersed in the liquid, respectively. Gas is forced through the capillaries and its flow rate and the back pressure is measured to calculate the DST of the liquid. However, liquids that dry fast and have inadequate resolubility dry inside the capillaries and substantially affect the DST (i.e. pressure) measurements.
While others have determined the surface tension of a liquid using DST measurements, no one has associated the DST (pressure) measurements with the drying rate of liquids or proposed this principle as a method to quantify the drying rate of liquids. The DST of liquids can be measured with relative ease. For example, U.S. Pat. No. 3,881,344 entitled “Monitor for Continuously Measuring Surface Tension of Liquids” discloses an apparatus for measuring the DST of a liquid in a flowing stream, whereby a capillary tube is dipped in the liquid and a gas bubble is formed on the end of the tube by forcing air down the tube. The pressure required to form the bubble at the capillary tip below the liquid surface is related to the surface tension of the liquid via the following expression:γ=(dΔP)/4where:                γ is the dynamic surface tension in dynes/cm or mN/m;        d is the effective diameter of the capillary tube in cm; and        ΔP is the excess of the inert gas pressure required to generate bubbles at the capillary tip immersed in the liquid at a given rate, in dynes/cm2.        
However, the association between this pressure (or the DST of the liquid) and the drying rate of a liquid has not been explored and developed.
Moreover, there exists a need to be able to quantitatively measure the drying rate of a liquid by virtue of monitoring the changes in electrical conductance and resistance associated with the liquid as it dries. Such a test is needed which would be industrially suitable for applications involving printing inks, paints, coatings, varnishes, lacquers, adhesives and the like. This would include gravure and flexographic inks, such as water based and solvent based liquid inks and for paper packaging and solvent based and water based inks for film. More specifically, evaluation of the degree of cure (i.e. drying) of an ultraviolet (UV) or electron beam (EB) cured ink and/or coating is a very important issue for all printers that use energy curable (EC) inks and, for example, coatings. Such evaluation can be done in the lab using very sophisticated instrumentation, such as chromatography and spectroscopy. However, there is currently no method and instrument presently available that would allow for quick and easy degree of cure measurement. The industrial test currently used (a solvent rub test) is off-line and is very subjective and irreproducible. There is a high demand in the market for an instrument and a method capable of quickly quantifying and determining the degree of cure for EC ink and/or coating formulations. In addition, no one has developed a way to quickly measure the drying rate of conventional or EC inks and coatings.